During thermal processing of materials, the control of the local temperature distribution in the processed bodies is essential. Although application of the basic methods described in this specification may be useful in other fields, the application described herein is directed to achieving temperature uniformity during thermal processing steps in manufacturing microelectronic devices on circular semiconductor wafers.
There is an economically motivated trend of using semiconductor wafers with larger radii. At the same time, the sizes of individual integrated devices are reduced. A significant increase in temperature ramping rates between individual thermal steps is necessary both for economic and technical reasons. At the same time, an ever-increasing temperature uniformity both from wafer to wafer and across the wafer radius is required. However, the increase in wafer size and in temperature ramping rates, both lead to a significant increase in radial temperature non-uniformity in each wafer during manufacture.
During heating, in a commonly used batch-type horizontal or vertical diffusion furnace (conventional furnace), temperature measurements show the lowest temperature occurring at the wafer center, which increases monotonically towards a maximum value, which is observed at the wafer edge.
During cooling down, the reverse effect is observed. Theoretically, this can be explained by the monotonic increase of the geometric factors (view factors) for radiative heat exchange between the wafer and the outside of the inter-wafer region (where the heater elements are located) with decreasing distance from the wafer center.
This situation is shown in FIG. 1. A plurality of semiconductor wafers or wafers 12 are located in parallel with distances of t therebetween. A tubular heater 14 surrounds the wafers 12. The radius of the wafers 12 is r. The area C is an effective heater area of the heater 14, which heats a point X at the distance x (0.ltoreq..times..ltoreq.r) from the center of one of the wafers 12". The area C includes a first area above R (R is a intersection point of the heater surface and imaginatry extention of wafer 12") which directly heats the upper surface of the semiconductor wafer 12", and a second area below R which directly heats the lower surface of the semiconductor wafer 12". As can be seen, a decrease in x causes a decrease in the area C because of the shadowing effects of the neighboring semiconductor wafers 12' and 12'". These shadowing effects cause the radial temperature non-uniformity.
In current technology, special wafer-boats with ring-shaped trays placed under each stacked wafer are used. The technology is described in detail in U.S. Pat. No. 5,297,956 (K. Yamabe et. al; Mar. 29, 1994) which is incorporated herein by reference. FIG. 2 shows a cross-sectional view of the ring-shaped trays 17, semiconductor wafers 12, and a boat 13. Typically the ring-shaped tray will be made of quartz or SiC. The effect of the ring is to draw heat from the wafer edge region during temperature ramping (or heating) and to supply heat during the cooling down period, thus compensating the higher rates of temperature change in the edge region compared to the center region of each wafer. The wafer boats with rings were found to significantly improve the temperature non-uniformity during temperature ramping.
However, the additional mass of the ring reduces the temperature ramping-rates on the wafers especially when approaching the holding temperatures. Thus the problem of delay times at the onset of heating or cooling periods and similarly when approaching the holding temperature arises. For achieving the same target ramping rate of the wafer edge, an increase in heater power and/or using a larger wafer spacing is necessary compared to the case of the described conventional wafer boat of FIG. 1. These effects become increasingly disadvantageous with increasing diameter of the wafers to be heat treated.